In paradise plaza there is a bunch of flower pots (the ones without trees in them) and break them over the zombies heads, if you run out simply run out of the area and come back for them to respawn.

Method 2:

Credit to this idea goes to youtube user VidGamiacUnlocked

Head to the food court at any time in the game and head to the left where is a giant stack of wooden barrels, you can take these and keep smashing them on zombies, every one that breaks is 1 towards this achievement.